Is it worth spending 1 day in Salt Lake City if you only have 3 days to split with Yellowstone National Park?

You can't tour the temple itself anyway, you can walk around the grounds, called Temple Square, and the capitol building of course is open to the public. But, honestly, its not much to see. Worth seeing once in your life, sure, but don't go too far out of your way or take too much time from your yellowstone trip. As for the Great Salt Lake? Don't even bother. It stinks, and is a big lake. Nothing much to see, cept for brine shrimp. Again, possibly worth seeing once in your life, but go for yellowstone, most definitely.